1. Field of the Invention
The present invention relates to an apparatus and a method for forming a digital image with use of light intensity modulation in an electrophotographic process.
2. Description of the Prior Art
In a digital printer and the like with use of an electrophotographic process, a laser beam is modulated according to multi-level image data to expose a photoconductor sensitized uniformly, to form an electrostatic latent image. The latent image is developed with toners, and the resultant toner image is transferred to a paper. Thus, the image data is reproduced on the paper.
In a method of forming a digital image, the intensity of laser beam is modulated according to multi-level image data to express gradation. In this method, the size of laser beam is kept constant.
However, the beam size varies with the type of laser diode used for the exposure. In U.S. patent application Ser. No. 671,963, the beam size of laser diode is coded, and the coding is performed in a factory before shipment. The beam size (code input) of a laser diode installed in an apparatus is detected, and the gradation correction data is corrected according to the detected beam size.
A latent image formed with the laser beam is an analog image, and the gradation characteristic depends largely on the size of laser beam. The size varies with the environment and the like of the electrophotographic process. Therefore, it is desirable that gradation characteristic is corrected according to the beam size.
The beam size can be changed for example by using a beam expander disclosed in Japanese Patent laid open Publication No. 68,316/1985. It can also be changed with a slit (Japanese Patent laid open Publication No. 72,979/1991) or with an electro-optic ceramic plate (Japanese Patent laid open Publication No. 136,018/1991). However, these methods are difficult to be used practically because the beam size and the change thereof are various according to the installation precision and the control method. Further, the shape of the beam does not necessarily become the same as the beam spot focused on the photoconductor when the beam is detected with a sensor located at a place different from the photoconductor.
Then, it is considered good that the beam size is corrected, without detecting the beam size, by deciding the effect of the beam size on the gradation characteristic from the potential of an exposed image on the photoconductor and the state of the image after the development. However, in this case, it is difficult to correct the effect of the beam size by separating the effect of the beam size from the changes of the photoconductor characteristic and the development characteristic.